The behavior of boron and fluorine introduced into the SiO2/Si system is investigated. Fluorine introduction increases sheet resistances of boron layers in silicon after heat-treatment in both nitrogen and oxygen atmospheres. Boron depth profiles reveal that fluorine causes boron to migrate into SiO2 from Silicon. This phenomenon can be explained if we assume that fluorine is incorporated into the SiO2 network together with boron to lower the viscosity of SiO2.